The present invention relates to a buoyancy device particularly, but not exclusively, for attachment to large structures requiring to be lifted, lowered, positioned and transported via the ocean.
Conventionally, when an offshore drilling structure comes to the end of its working life, it is decommissioned. In the past, it was thought that decommissioning could entail sinking the drilling structure at the point where it once stood.
However, amongst other factors, environmental factors have recently increased the need for offshore drilling structures to be moved to shallower and calmer waters, or back on land so that the drilling structures can be dismantled safely.
Previously, moving the drilling structures has involved the use of flat back barges, onto which the drilling structures are hauled. However, these barges are expensive and costly in terms of man power requirements.
Our previous PCT application GB97/01350 concerns a type of buoyancy device upon which the present invention seeks to improve.
According to one aspect of the present invention there is provided a buoyancy device comprising at least one buoyancy member optionally connected to a coupling member for attachment of the coupling member to a structure to be floated or sunk, wherein the buoyancy member is inflatable and comprises a substantially flexible portion and one or more end portions, the or each end portion being releasably attached to the flexible portion.
The end portions may be dome-shaped, and the flexible portion may be tubular and may extend between two end portions. The dome-shaped end portions may be arranged in a first configuration in which their convex portions face one another, or may be arranged in a second configuration in which their concave portions are facing one another. The end portions may be connected together by any suitable connector, eg by straps or chains etc. The connectors may have tensioning devices incorporated in order to alter the tension and this can in some embodiments be carried out when the end portions are connected in situ. The end portions can have ports to allow access to the interior of the device.
The end portions can be connected to the flexible portions, for instance, at a section of overlap between the two portions. The flexible portion preferably overlies a respective end portion at said overlap, but this may be reversed. The flexible portion can be hooked onto the end portion, or in a preferred embodiment has bands extending around the flexible portion at the overlap, so as to tighten the band around the flexible portion and compress it against the end portion. One or more bands can be provided, preferably of steel or plastics material, and preferably of an inextensible material.
Another aspect of the invention provides a buoyancy device comprising an array of buoyancy members connected to a coupling member for attachment to a structure to be floated or sunk, wherein the coupling member is disposed at one or more ends of the buoyancy members.
In this embodiment the coupling member can be a plate extending between at least two buoyancy members at an end thereof. The plate can be a simple bar extending between two members which can be adjacent, or can be a lattice which connects a number of adjacent or nonadjacent members in the array.
According to a third aspect the present invention provides a buoyancy device comprising a coupling member for attachment to, or arrangement around, one or more members of a structure to be floated or sunk, the coupling member having at least one coupling point for coupling to a buoyancy member, wherein the coupling member permits the transfer of loads from the buoyancy. device to the member(s) of the structure.
Preferably, the coupling member has a plurality of coupling points for coupling to a plurality of buoyancy members, and preferably to a plurality of respective buoyancy members.
Typically, the coupling member comprises a bore which locates around at least a portion of the outer surface of the member of the structure. Preferably, the said bore is shaped to substantially correspond to the shape of the outer surface of the member of the structure, and where the member is a tubular, the said bore is preferably substantially cylindrical.
The coupling member is preferably arranged to be substantially tubular, and typically, the coupling points are arranged on the outer surface of the coupling member.
The coupling member is preferably arranged to clamp around the outer circumference of the member of the structure, and more preferably, the coupling member is releasably clamped to the member of the structure. Typically, the coupling member is provided with a hinge and/or a locking mechanism to permit the coupling member to be coupled to, and released from, the member of the structure.
A plurality of buoyancy members may be substantially equi-spaced around at least a portion of the circumference of the coupling member, and may be equi-spaced around the whole of the circumference of the coupling member. Alternatively, the spacing between the respective buoyancy members may be variable.
According to a fourth aspect of the present invention there is provided a buoyancy device comprising a buoyancy member for attachment to a structure to be floated or sunk, the buoyancy member comprising a restriction device and an inflatable member located substantially within the restriction device, such that the restriction device restricts inflation of the inflatable member.
The inflatable member can be a bag, a diaphragm or a bladder.
Preferably, the restriction device permits the ingress and/or egress of fluid from within the device, and typically, the restriction device has at least one aperture to permit fluid to flow therethrough. Typically, the buoyancy device is arranged such that water located outwith the restriction device can enter at least a portion of the restriction device and act upon the outer surface of the bag, diaphragm or bladder.
Typically, the restriction device is of a substantially tubular nature.
Preferably, one end of the inflatable bag is secured to one end of the restriction device, and typically, the said one end of the bag is coupled to a fluid inlet and/or outlet port. The bag is preferably formed from a substantially flexible material, and the restriction device is preferably formed from a substantially rigid material.
Alternatively, one end of the inflatable diaphragm or bladder is secured to a portion of the restriction device, which may be the interior of the device and preferably is a portion substantially at the mid-point of the interior of the restriction device, and typically, the said one end of the diaphragm or bladder is in fluid communication with a fluid inlet and/or outlet port. The diaphragm or bladder is preferably formed from a substantially flexible material, and the restriction device is preferably formed from a substantially rigid material. Typically, the one end of the diaphragm or bladder is secured to the restriction device by means of a clamping ring, which typically forms a sandwich of the said one end with the inner surface of the restriction device.
Preferably, the bag is substantially tubular when inflated.
Typically, the inflatable bag is further restrained, along it""s longitudinal axis by one or more movement restraining devices, wherein said devices may comprise a substantially elastic ring secured to the outer surface of the bag and preferably secured around at least a portion of the outer circumference of the bag, and a tie which preferably acts between the elastic ring and the interior wall of the restraining device.
Preferably, there are a plurality of apertures formed in the sidewall of the restraining device, and which may be formed in the sidewall along the length of the restraining device. Alternatively, or in addition, there may be one or more apertures formed in an end of the restraining device.
The invention also provides a buoyancy device having the one or more buoyancy members containing an incompressible buoyancy material.
Another aspect of the invention provides a buoyancy device comprising a chamber having a variable buoyancy, and means to vary the buoyancy of said chamber.
The chamber preferably comprises an extensible portion which can be expanded or contracted to increase or decrease the buoyancy of the chamber.
The means to vary the buoyancy of the chamber can be a gas cylinder optionally located in the chamber itself, although this is not necessary. The gas cylinder can be charged with any suitable type of gas more buoyant than the fluid in which the device is to be used. The device can comprise an array of such chambers, or a mixture of variable-buoyancy chambers and other buoyancy members which do not have variable buoyancy means. The device is preferably arranged with an array of buoyancy members comprising an inner ring of members and an outer ring of members, the buoyancy of the outer ring members optionally being non-variable, and that of the inner ring members being preferably variable, but some other arrangement of variable/non-variable members may be used. The buoyancy of the variable members is preferably variable from outwith the device, and for this purpose the device may have external control means.
The buoyancy members can be inflatable and have a substantially flexible portion and one or more end portions, the or each end portion being releasably attached to the flexible portion as previously described.
Typically, the coupling member is a tubular member and the buoyancy members may be coupled along the length of the tubular member, or may be coupled to the ends of the tubular member.
Typically, the tubular member is substantially horizontal in use, when coupled to the structure required to be lifted, and after the buoyancy members have been inflated.
Typically, the tubular member is coupled to the structure when the buoyancy members are deflated.
Preferably, the tubular member is coupled to the structure in an initially horizontal plane.
Alternatively, the tubular member is initially coupled to the structure such that longitudinal axis of the tubular member is approximately 45xc2x0 to the horizontal plane.
An inflation means inlet and/or a deflation means outlet may be located at one or both of the end portions.
The buoyancy device may further comprise a pressure sensor to sense the pressure in the surrounding water, and may further comprise a displacement sensor to measure the displacement of the buoyancy device, and may further comprise an acceleration sensor to measure the acceleration of the buoyancy device.
A pressure sensor may be provided to sense the pressure within each inflatable member.
Typically, there is provided at least one valve to allow regulation of the pressure within an inflatable member. There may be at least one inflation valve to allow the pressure of air within each inflatable member to be increased and there may be at least one deflation valve to allow the pressure of air within each inflatable member to be decreased.
Preferably, should one or more of the inflatable members deflate, the pressure within the remaining inflatable members may be increased to compensate for the deflated member(s). Preferably, the inflatable members are restrained from over-inflation by a restraining device.
Preferably, the buoyancy device further comprises a control system to allow variation of its buoyancy. More preferably, a number of inflatable members are provided with a control system to allow variation of the buoyancy of the inflatable members.
Preferably, the control system is connected to, and reads signals from, the surrounding water pressure sensor, the inflatable member pressure sensor, the displacement sensor and the acceleration sensor. More preferably, the control system varies the buoyancy of the inflatable member in response to the signals read.
One or more of the inflatable members may be inflated by air. Alternatively, one or more of the inflatable members are inflated with an incompressible material having a density less than that of the surrounding water. Typically, the inflatable members are inflatable bags.
The features of one of the aspects of the invention can readily be combined with features from another aspect of the invention.